Field of the Invention
The present invention relates to renewable energy and more particularly to control of wind turbines.
Description of the Prior Art
A wind turbine converts the kinetic energy from the wind into electrical or mechanical energy. For conversion of the wind energy to electrical energy, the wind turbine is made up of the following elements:
A tower allows a rotor to be positioned at a sufficient height to enable motion thereof (necessary for horizontal-axis wind turbines) or allowing the rotor to be positioned at a height enabling it to be driven by a stronger and more regular wind than at ground level. The tower generally houses part of the electrical and electronic components (modulator, control, multiplier, generator, etc.).
A nacelle is mounted at the top of the tower, housing mechanical, pneumatic and some electrical and electronic components necessary to operate the machine. The nacelle can rotate to adjust the machine to the correct wind direction.
A rotor is fastened to the nacelle, comprising blades (generally three) and the nose of the wind turbine. The rotor is driven by wind energy and it is connected by a mechanical shaft, directly or indirectly (via a gearbox and mechanical shaft system), to an electrical machine (electrical generator) that converts the energy recovered to electrical energy.
A transmission having two shafts (mechanical shaft of the rotor and mechanical shaft of the electrical machine) is connected by a transmission (gearbox).
Since the beginning of the 1990s, there has been renewed interest in wind power, in particular in the European Union where the annual growth rate is about 20%. This growth is attributed to the benefit of carbon-emission-free electricity generation. In order to sustain this growth, the energy yield of wind turbines still has to be improved. The prospect of wind power production increase requires developing effective production tools and advanced control tools in order to improve the performance of the machines. Wind turbines are designed to produce electricity at the lowest possible cost. They are therefore generally built to reach their maximum performance at a wind speed of approximately 15 m/s. It is in fact unnecessary to design wind turbines that maximize their yield at higher wind speeds, which are not common. In the case of wind speeds above 15 m/s, it is necessary to not capture part of the additional energy contained in the wind to avoid damage to the wind turbine. All wind turbines are therefore designed with a power regulation system.
For this power regulation, controllers have been designed for variable-speed wind turbines. The purpose of the controllers is to maximize the electrical power which is recovered, to minimize the rotor speed fluctuations and to minimize the fatigue and the extreme moments of the structure (blades, tower and platform).
Linear controllers have been widely used for power control which control the blade pitch angle (orientation of the blades). These include techniques using PI (proportional integral) and PID (proportional integral derivative) controllers, LQ (linear quadratic) control techniques and strategies based on robust linear controls.
However, the performance of these linear controllers is limited by the highly non-linear characteristics of the wind turbine. First a strategy based on non-linear controls was developed. Such a strategy is for example described in the document: Boukhezzar B., Lupu L., Siguerdidjane H., Hand M. “Multivariable Control Strategy for Variable Speed, Variable Pitch Wind Turbines” Renewable Energy, 32 (2007) 1273-1287.
None of these strategies however uses the incident wind speed which is a fundamental element for the aerodynamic phenomena that govern the wind turbine. To take this component into account, initial work was performed with a measurement of the wind speed. This work shows that the productivity of a wind turbine and the life thereof can be significantly increased through innovative strategies using the wind speed.
This technique unfortunately requires a sensor that is expensive and is not very accurate. To take the incident wind speed into account without a sensor, an estimation of this speed can be performed to use this data in the control. Further work has been conducted to this end using a Kalman filter, which is described in the document: Boukhezzar B., Siguerdidjane H., “Nonlinear Control of Variable Speed Wind Turbine Without Wind Speed Measurement” IEEE Control and Decision Conference (2005). This method is not sufficiently accurate because the wind reconstruction is poorly representative. Indeed, according to this method, the wind is not structured and is considered as white noise, which is not the case experimentally.